In retrospect, the AVR plays 440 Hz at a rather low volume. It's a bit
hard to hear and impossible to hear when shown off in a Youtube
demonstration. Don Froulas changed it to 1004 Hz for better audibility
and as a reference to the telco digital milliwatt test signal frequency.

It only works if the first keystroke made is holding 2600 to shift to
playback mode and then the second being a key to play back a memory. At
this point, pressing and holding a key will abort playback after the
current one is finished. Subsequence playbacks are not aborted. Nor are
playbacks aborted if one of the twelve other keys are pressed before shifting
into playback mode.